"The bailout agreement needs to be signed otherwise we will be out of the markets, out of the euro. The situation will be much worse." That was the unhappy new year message from the Greek government and probably should not be regarded as another attempt to coerce the voters into accepting deeper austerity measures. Rather, it sounded like a plea to private sector holders of Greece's bonds to please hurry up and accept 50% haircuts on their investments.

The so-called PSI (private sector involvement) deal was announced in October but still has not been agreed, despite frequent optimistic noises from Athens. This is fast becoming a serious headache, since the next bailout, which has to happen before a €14bn (£11.7bn) Greek bond matures at the end of March, will stall without it and Greece would indeed be on its way out of the euro.

The problem is twofold. First, some holders of Greek debt will prefer to decline the "voluntary" offer of a haircut and insist instead on being paid on time and in full. This is not an option available to mainstream eurozone banks, which must do as their governments ask, but there are plenty of hedge funds and others that would not care about being cast as freeriders.

Thus last month's International Monetary Fund report on Greece, which said the PSI deal must happen "expeditiously," added that Greek and eurozone authorities should consider unspecified "tools" to encourage "near-universal creditor participation." Anything less than 95% participation would surely be a flop, and a dangerous flop since the official projections of Greek debt levels are already absurdly tight: even with a 50% haircut on private sector debt, the IMF calculates that Greece's debts would still be 120% of its GDP by 2020.

It is any wonder, then, that reports have surfaced that the IMF is arguing privately that haircuts for bondholders need to be greater than 50% to get overall Greek debt back to a level that can be deemed (for official purposes) sustainable. If those reports are correct, the screw is tightening further since achieving full-ish participation in 65% or even 75% haircuts might prove impossible. Having started at 21% last July, investors would naturally conclude that they're only one step away from 100%. At that point, the idea of Greece exiting the euro would be firmly back on the agenda.

No wonder, too, that economists' predictions for the course of the euro crisis in 2012 contain so many get-out clauses. In the case of Greece, it's hard to know whether October's debt deal can even survive the next three months.